This invention relates to an improved process for making a nonwoven fabric from a fibrous web.
Two basic processes are described in the literature for impinging fine columnar streams of liquid on a fibrous web to convert it by fiber entanglement into a nonwoven fabric. In one process, disclosed in U.S. Pat. No. 3,493,462 and 3,560,326, the apertured support is of such a fine mesh that the fabric has no pattern of apertures arising from the support but does have a pattern of lines of entangled fibers (jet tracks) reflecting the paths of impingement of the fine columnar streams across the surface of the fabric.
In the other process, disclosed in U.S. Pat. No. 3,485,706, for a given area weight web, the larger size of the apertures in the support in combination with the larger solid regions in the support cause apertures to form in the fabric in regions corresponding to the solid regions of the support. While impingement of the fine columnar streams of liquid on the web cause the fiber entanglement that holds the resultant fabric together, it is the apertured pattern appearance that is desired for the fabric.
In the practice of both processes, the orifices from which the streams of liquid issue have been arranged in a single row per bank. A bank is a series of orifices supplied with liquid by a single manifold. A series of banks of orifices in single rows have been used so as to provide a stepwise increase in impact pressure of the streams on the fibrous web. As shown in FIG. 41 of U.S. Pat. No. 3,485,706, these banks of manifolds (98) have been spaced far apart.
The streams of liquid have been columnar, i.e., as close to nondiverging as possible, so as to minimize the area of impingement of the stream on the web and thereby concentrate the entanglement force of the stream on the web. U.S. Pat. No. 3,403,862 discloses apparatus for improving columnarity of the streams. This patent also discloses that the spacing between stream orifices should "be sufficient to avoid interaction between adjacent streams" (column 4, lines 23-25). Interaction of the streams on their way to impinge on the web would increase the area of impingement of the streams on the web, thereby decreasing the entanglement force and efficiency of operation.
In the practice of both of the aforesaid processes the orifices in the single row in each bank have been spaced close together in order to get complete coverage of the streams over the face of the web, while avoiding the aforesaid interaction between adjacent streams. By way of example, the 5 mil (0.127 mm) diameter orifice has been used at a frequency of 40 orifices/inch (15.7/cm) which corresponds to a spacing between orifices (center-to-center) in the row of 25 mils (0.635 mm). This complete coverage has provided a jet track type fabric wherein the jet tracks are close together and uniformly faint in appearance so as to give this type of fabric a uniform appearance. With respect to the fabric having a pattern of apertures therein, even this complete coverage has not always provided the uniform pattern of apertures in the fabric. Occasionally, lines of fuzzy or indistinct apertures alternate with lines of distinct apertures in the fabric, which lines are believed attributable to interference between certain streams and certain repeat points in the topography of the apertured support for the web when such support is a screen. Such lines, which run in the direction of the passage of the web beneath the streams, can be called interference lines. These interference lines which vary in frequency and intensity from screen to screen have been erasable by passage of the web through a final row of streams spaced closer together than the proceding rows, e.g., using an orifice frequency of 60/inch (23.6/cm) for the 5 mil (0.127 mm) diameter orifice, whereby the resultant pattern of apertures in the fabric is uniform.
While these processes are effective in producing strong nonwoven fabrics by fiber entanglement alone, the problem has arisen of how to increase the efficiency of the liquid. While the liquid is recycled, nevertheless, the energy cost in pumping the liquid is high. This efficiency could manifest itself as the making of stronger fabric with the same amount of water; or using less water to get a fabric of equivalent strength; or using lower area weight fibrous web and getting a fabric of equivalent strength, which also increases the rate of production of the fabric.